Spectroscopic analysis and conventional and phase-sensitive flow cytometry were used to compare changes in PI and EB fluorescence intensity and lifetime bound to DNA and fixed Chinese hamster ovary (CHO) cells in the presence of D2O versus phosphate buffered saline (PBS). A two-fold enhancement of fluorescence intensity of PI and EB bound to fixed-CHO cells in D2O was noted as well as a 5 ns increase in PI and EB fluorescence lifetimes in D2O. We have recently used D2O in studies on the lifetime of EB bound to human diploid fibroblasts located at the G1/S phase transition point. Results indicate that alterations chromatin states during the entry into S phase structures lead to alterations in the fluorescence intensity and fluorescence lifetime of the DNA-intercalated EB.